aead: have NewAead borrow the key

In many AEAD implementations we pass the key directly onto
`NewBlockCipher`, e.g. in the `aes-gcm` crate:

https://github.com/RustCrypto/AEADs/blob/af9926e/aes-gcm/src/lib.rs#L183

This makes an unnecessary copy of the key which therefore necessitates
zeroing it out.

If we borrow the key at the time the cipher is initialized, we can avoid
making this copy.

aead/src/lib.rs

@@ -74,7 +74,7 @@ pub trait NewAead {
     type KeySize: ArrayLength<u8>;
 
     /// Construct a new stateful instance for the given key.
-    fn new(key: GenericArray<u8, Self::KeySize>) -> Self;
+    fn new(key: &GenericArray<u8, Self::KeySize>) -> Self;
 }
 
 /// Authenticated Encryption with Associated Data (AEAD) algorithm.